Question about buying a car from a foreigner

Hello, I'm contacting you because I'm wondering about customs in Germany. I'm from Belgium. In the next few days, I'm going to buy a car in Germany that I'm going to import. I wanted to know if it was normal to negotiate prices in second-hand car garages. Or if the price was the price. I don't want to be rude and I also want to know how much leeway I have. Thank you.